MCP tools that return `string[]` get different schema from `SomethingElse[]`

[AArnott]

A tool that returns an array of things should have a consistent schema for the array itself, leaving the changes to only the type of thing being returned.

When I call a method that returns Animal[], this is the result:

{
 "content": [
 {
 "type": "text",
 "text": "[{\"name\":\"Bear\",\"weight\":500},{\"name\":\"Elephant\",\"weight\":6000}]"
 }
 ],
 "isError": false
}

Yet when I call a method that returns string[], this is the result:

{
 "content": [
 {
 "type": "text",
 "text": "Bear"
 },
 {
 "type": "text",
 "text": "Elephant"
 }
 ]
}

Note the subtle yet significant difference of where the 2 element array is.

• For Animal[] we get a content array of 1, and a JSON-formatted string containing an array of 2.
• For string[] we get a content array of 2, and no JSON-formatted string at all.

@stephentoub found that the MCP server library special cases IEnumerable<string> leading to this inconsistency:

csharp-sdk/src/ModelContextProtocol.Core/Server/AIFunctionMcpServerTool.cs

Lines 273 to 277 in 329f848

	IEnumerable<string> texts => new()
	{
	Content = [.. texts.Select(x => new TextContentBlock { Text = x ?? string.Empty })],
	StructuredContent = structuredContent,
	},

Repro server:

[McpServerToolType]
public class TimeTool
{
 [McpServerTool, Description("Gets a list of interesting animals.")]
 public static Animal[] GetAnimals(
 IMcpServer server,
 CancellationToken cancellationToken)
 {
 return [new Animal("Bear", 500), new Animal("Elephant", 6000)];
 }

 [McpServerTool, Description("Gets a list of interesting animals.")]
 public static string[] GetAnimalStrings(
 IMcpServer server,
 CancellationToken cancellationToken)
 {
 return ["Bear", "Elephant"];
 }

 public record Animal(string Name, int Weight);
}